Downhole digital power amplifier for a measurements-while-drilling telemetry system

Abstract

A downhole digital power amplifier comprises a device which, through digital means, provides a sinusoidal, variable frequency, variable power and variably phase-shift modulated output for the transmission of data from a downhole sensor arrangement to an uphole receiver. Selection and variation of frequency, power and modulation are made through digital inputs. The device provides these functions at extra low frequencies (ELF) low-to-high power output, and varying phase-shift keying. The downhole digital power amplifier (68) comprises, in a preferred embodiment, an input shift register (81) for receiving a digital bit stream, a programmable frequency divider (82), a dead man timer circuit (84), a counter (86), a sync circuit (88), a PROM (90) (programmable read-only memory) for generating a digital sinusoidal output, a DAC (92) (digital-to-analog converter) for converting the digital sinusoidal output to an analog sinusoidal output, a power supply control circuit (94), an analog divider (96), and a conventional power amplifier (98). The dead man timer circuit detects normal operation and a fault condition, and controls the power supply control circuit to "power down" the amplifier when reception of a digital bit stream is not imminent during normal operation, and controls the power supply control circuit to periodically generate a "failure informant" message during a fault condition.

We claim: 
     
       1. A downhole digital power amplifier for use in downhole telemetry applications, wherein the downhole digital power amplifier has the capability of selecting frequency, power and modulation modes, said downhole digital power amplifier comprising: digital input means for selecting at least one of frequency and power, and for additionally selecting a modulation mode of the downhole digital power amplifier;   generating means for generating a sinusoidal output characterized by said selected at least one of said frequency and power, and modulated in accordance with said selected modulation mode; and   transmitting means for transmitting said generated sinusoidal output from the downhole digital power amplifier through the earth to the earth's surface.   
     
     
       2. The downhole digital power amplifier for use in downhole telemetry applications as defined in claim 1, wherein: frequency is selected and a digital bit stream is provided to said digital input means, said digital input means comprising a shift register having a plurality of bits for indicating said selected frequency.   
     
     
       3. The downhole digital power amplifier for use in downhole telemetry applications as defined in claim 1, wherein: power is selected and a digital bit stream is provided to said digital input means, said digital input means comprising a shift register having a plurality of bits for indicating said selected power.   
     
     
       4. The downhole digital power amplifier for use in downhole telemetry applications as defined in claim 1, wherein: a digital bit stream is provided to said digital input means, said digital input means comprising a shift register;   said digital input means comprising means for providing a clock signal, and a programmable frequency divider for receiving and frequency-dividing said clock signal in accordance with a frequency-indicating portion of said digital bit stream so as to develop a frequency divider output;   said digital input means further comprising a counter responsive to said frequency divider output from said programmable frequency divider for counting in accordance therewith, said counter counting to a zero state and issuing a corresponding zero output;   said digital input means further comprising a sync circuit responsive to said zero output from said counter for issuing a corresponding output; and   said operating means comprising a programmable read-only memory for issuing digital outputs corresponding in value to successive amplitude values of a sinusoidal waveform,   said programmable read-only memory being responsive to said corresponding output of said sync circuit for changing from a positive-going portion of said sinusoidal waveform to a negative-going portion of said sinusoidal waveform only when said counter has achieved said zero state.   
     
     
       5. The downhole digital power amplifier for use in downhole telemetry applications as defined in claim 4, said generating means further comprising: a digital-to-analog converter connected to said programmable read-only memory for receiving and converting said digital outputs to analog form so as to provide a corresponding analog output; and   an analog divider responsive to said corresponding analog output of said digital-to-analog converter and to a further portion of said digital bit stream for dividing said analog output to produce an analog divided output comprising said sinusoidal output;   said transmitting means comprising a power amplifier for receiving said analog divided output and responsive thereto for issuing an amplified output for transmission to the earths surface.   
     
     
       6. The downhole digital power amplifier for use in downhole telemetry applications as defined in claim 1 and further comprising: detecting means for detecting a fault condition and responsive thereto for periodically generating a failure informant signal for transmission   
     
     
       7. The downhole digital power amplifier for use in downhole telemetry applications as defined in claim 6, wherein: said detecting means comprises a dead man timer circuit for receiving a digital input and responsive thereto for detecting normal operation or a fault condition of said downhole digital power amplifier, and responsive to detection of said fault condition for generating a power-off signal of a first duration periodically interrupted by a power-on signal of a relatively small second duration; and   a power supply control circuit responsive to said power-on signal and said power-off signal for applying and not applying, respectively, power to said generating means, whereby to generate said failure informant signal.   
     
     
       8. The downhole digital power amplifier for use in downhole telemetry applications as defined in claim 7, wherein: said dead man timer circuit receives a further digital input indicating when reception of said digital input is to be expected, said dead man timer circuit being responsive to detection of said normal operation for generating said power-off signal when reception of said digital input is not expected and for generating said power-on signal when reception of said digital input is expected.   
     
     
       9. The downhole digital power amplifier for use in downhole telemetry applications as defined in claim 4, wherein said sync circuit comprises a D-type flip-flop having a data input, a clock input for receiving said zero output from said counter, and a Q output for issuing said corresponding output, said data input being connected to selector means for selecting the digital bit stream under normal, non-fault conditions of operation of the downhole digital power amplifier, and for selecting the Q output of the D-type flip-flop under abnormal, fault conditions of operation of the downhole digital power amplifier, whereby setting/resetting of the D-type flip-flop is controlled by the digital bit stream and the zero output of the counter under normal conditions, and the D-type flip-flop functions as a divide-by-two circuit in response to the zero output of the counter under fault conditions. 
     
     
       10. A digital power amplifier for use in telemetry applications, wherein the downhole digital power amplifier has the capability of selecting frequency, power and modulation modes, said downhole digital power amplifier comprising: digital input means for selecting at least one of frequency and power, and for additionally selecting a modulation mode of the digital power amplifier; and   generating means for generating a sinusoidal output characterized by said selected at least one of said frequency and power, and modulated in accordance with said selected modulation mode.   
     
     
       11. The digital power amplifier for use in telemetry applications as defined in claim 10, wherein frequency is selected and a digital bit stream is provided to said digital input means, said digital input means comprising a shift register having a plurality of bits for indicating said selected frequency. 
     
     
       12. The digital power amplifier for use in telemetry applications as defined in claim 10, wherein power is selected and a digital bit stream is provided to said digital input means, said digital input means comprising a shift register having a plurality of bits for indicating said selected power. 
     
     
       13. The digital power amplifier for use in telemetry applications as defined in claim 10, wherein: a digital bit stream is provided to said digital input means, said digital input means comprising a shift register;   said digital input means comprising means for providing a clock signal, and a programmable frequency divider for receiving and frequency-dividing said clock signal in accordance with a frequency-indicating portion of said digital bit stream so as to develop a frequency divider output;   said digital input means further comprising a counter responsive to said frequency divider output from said programmable frequency divider for counting in accordance therewith, said counter counting to a zero state and issuing a corresponding zero output;   said digital input means further comprising a sync circuit responsive to said zero output from said counter for issuing a corresponding output; and   said generating means comprising a programmable read-only memory for issuing digital outputs corresponding in value to successive amplitude values of a sinusoidal waveform,   said programmable read-only memory being responsive to said corresponding output of said sync circuit for changing from a positive-going portion of said sinusoidal waveform to a negative-going portion of said sinusoidal waveform only when said counter has achieved said zero state.   
     
     
       14. The digital power amplifier for use in telemetry applications as defined in claim 10, wherein said sync circuit comprises a D-type flip-flop having a data input, a clock input for receiving said zero output from said counter, and a Q output for issuing said corresponding output, said data input being connected to selector means for selecting the digital bit stream under normal, non-fault conditions of operation of the digital power amplifier, and for selecting the Q output of the D-type flip-flop under abnormal, fault conditions of operation of the digital power amplifier, whereby setting/resetting of the D-type flip-flop is controlled by the digital bit stream and the zero output of the counter under normal conditions, and the D-type flip-flop functions as a divide-by-two circuit in response to the zero output of the counter under fault conditions. 
     
     
       15. The digital power amplifier for use in telemetry applications as defined in claim 13, said generating means further comprising: a digital-to-analog converter connected to said programmable read-only memory for receiving and converting said digital outputs to analog form so to provide a corresponding analog output;   an annalog divider responsive to said corresponding analog output of said digital-to-analog coverter and to a further portion of said digital bit stream for dividing said analog output to produce an analog divided output; and   a power amplifier for receiving said analog divided output and responsive thereto for issuing an amplified output comprising said sinusoidal output.   
     
     
       16. The digital power amplifier for use in telemetry applications as defined in claim 10, further comprising: detecting means for detecting a fault condition and responsive thereto for periodically generating a failure informant signal for transmission.   
     
     
       17. The digital power amplifier for use in telemetry applications as defined in claim 16, wherein: said detecting means comprises a dead man timer circuit for receiving a digital input and responsive thereto for detecting normal operation or a fault condition of said digital power amplifier, and responsive to detection of said fault condition for generating a power-off signal of a first duration periodically interrupted by a power-on signal of a relatively small second duration; and   a power supply control circuit responsive to said power-on signal and said power-off signal for applying and not applying, respectively, power to said generating means, whereby to generate said failure informant signal.   
     
     
       18. The digital power amplifier for use in telemetry applications as defined in claim 17, wherein: said dead man timer circuit receives a further digital input indicating when reception of said digital input is to be expected, said dead man timer circuit being responsive to detection of said normal operation for generating said power-off signal when reception of said digital input is not expected and for generating said power-on signal when reception of said digital input is expected.